1. Field of the Invention
The present invention relates to a device for controlling an internal combustion engine by using a NOx purifying catalyst to reduce NOx (nitrogen oxides) in the exhaust gas. More particularly, the invention relates to a device for controlling an internal combustion engine capable of estimating the amount of NOx emission within short periods of time maintaining high precision and realizing improved control performance without increasing the cost that results when a memory having a large capacity is used.
2. Prior Art
Devices for controlling internal combustion engines of this kind have heretofore been provided with NOx amount estimating means for estimating the amount of NOx adsorbed by a NOx adsorbing agent as taught in, for example, Japanese Patent No. 2586739.
FIG. 3 is a block diagram illustrating the constitution of a conventional device that is adapted to a gasoline engine.
To avoid complexity, here, the description deals with one cylinder only. It should, however, be noted that the same constitution applies to plural cylinders.
In FIG. 3, an internal combustion engine 1 includes a piston 2, a combustion chamber 3, a spark plug 4, an intake valve 5, an intake port 6, an exhaust valve 7 and an exhaust port 8.
The intake port 6 is coupled to a surge tank 10 through a corresponding intake pipe 9 which is provided with a fuel injection valve 11 for injecting fuel into the intake port 6.
The surge tank 10 is coupled to an air cleaner 13 through an intake duct 12 in which a throttle valve 14 is disposed. The intake duct 12 is further provided with an air flow sensor (not shown) for detecting the amount of the air taken in.
On the other hand, the exhaust port 8 is connected, through an exhaust manifold 15 and an exhaust pipe 16, to a casing 18 in which a NOx adsorbing agent 17 is contained.
The NOx adsorbing agent 17 adsorbs NOx in the exhaust gas and works as a NOx purifying catalyst.
An electronic control unit (ECU) 30 comprises a digital computer which includes a ROM 32, a RAM 33, a CPU 34, an input port 35 and an output port 36 which are connected to each other through a bidirectional bus 31, as well as A/D converters 37, 38 inserted on the input side of the input port 35 and drive circuits 39 inserted on the output side of the output port 36.
A pressure sensor 19 is mounted in the surge tank 10 to generate an output voltage in proportion to an absolute pressure in the surge tank 10. An output voltage of the pressure sensor 19 is fed to the input port 35 through the A/D converter 37.
An air-fuel ratio sensor 25 is mounted on the exhaust pipe 16. An output voltage of the air-fuel ratio sensor 25 is fed to the input port 35 through the A/D converter 38.
Further, a known EGR pipe (not shown) is provided between the exhaust pipe 16 and the intake pipe 9 to recirculate part of the exhaust gas. The EGR pipe is provided with an EGR valve for adjusting the EGR amount.
An idle switch 20 is attached to the throttle valve 14 to detect the idle opening degree of the throttle valve 14. An output signal of the idle switch 20 is input to the input port 35. Similarly, an output signal (engine rotational speed Ne) of a rotational speed sensor 26 is fed to the input port 35.
The operation of the conventional device shown in FIG. 3 will be briefly described below with reference to FIGS. 4 and 5. The control operation of the conventional device is as disclosed in detail in the above-mentioned patent publication, and is not described here.
The CPU 34 in the ECU 30 constitutes NOx amount estimating means in cooperation with the ROM 32 and RAM 33, and estimates the amount of NOx adsorbed by the NOx adsorbing agent 17.
It is difficult to directly detect the amount of NOx adsorbed by the NOx adsorbing agent 17. Therefore, the amount of NOx in the exhaust gas emitted from the engine 1 is found to estimate the amount of NOx adsorbed by the NOx adsorbing agent 17 from the amount of NOx in the exhaust gas.
In general, the amount of the exhaust gas emitted from the engine 1 per a unit time increases with an increase in the engine rotational speed Ne. Accordingly, the amount of NOx emitted from the engine 1 per a unit time increases with an increase in the engine rotational speed Ne.
Further, as the engine load increases (i.e., as the absolute pressure PM in the surge tank 10 increases), the amount of the exhaust gas emitted from the combustion chamber 3 increases and the combustion temperature increases. As the engine load increases (absolute pressure PM in the surge tank 10 increases), therefore, the amount of NOx emitted from the engine 1 per a unit time increases.
FIG. 4 is a diagram illustrating the amount of NOx emitted from the engine 1 per a unit time, and wherein the values found through experiment are related to the absolute pressure PM (ordinate) in the surge tank 10 and the engine rotational speed Ne (abscissa).
In FIG. 4, the continuous curves represent the same amounts of NOx.
As shown in FIG. 4, the amount of NOx emitted from the engine 1 per a unit time increases with an increase in the absolute pressure PM in the surge tank 10 and with an increase in the engine rotational speed Ne.
The amounts of NOx shown in FIG. 4 have been stored in advance in the ROM 32 in the form of map data N11 to Nij shown in FIG. 5.
The map data shown in FIG. 5 vary depending upon other various operating conditions. When it is attempted to correctly find the amount of NOx by operating the map, a large amount of memory capacity is necessary driving up the cost.
According to the conventional device of controlling the internal combustion engine as described above, the data used by the NOx amount estimating means in the ECU 30 are stored as map data N11 to Nij as shown in FIG. 5. Therefore, the map data must be formed for every operating condition of the engine 1 and must be stored in the ROM 32, requiring laborious work and extended periods of time and driving up the cost.
The present invention was accomplished in order to solve the above-mentioned problem, and has an object of providing a device for controlling an internal combustion engine by estimating the amount of NOx emission within short periods of time maintaining high precision and improving control performance without the need of storing great amounts of map data in the ROM and, hence, without driving up the cost.
A device for controlling an internal combustion engine according to the present invention comprises:
an air flow sensor provided in an intake pipe of the internal combustion engine to detect the amount of the intake air;
temperature detector means and pressure detector means for detecting the temperature and the pressure of the air taken in by the internal combustion engine;
air-fuel ratio detector means provided in the exhaust pipe of the internal combustion engine and for detecting the air-fuel ratio in the exhaust gas;
EGR rate detector means for detecting the EGR rate of the exhaust gas recirculated into the intake air;
a NOx purifying catalyst provided in the exhaust pipe of the internal combustion engine;
NOx operation means for estimating the amount of NOx in the exhaust gas from a theoretical formula and an empirical formula based upon the amount of the intake air, temperature and pressure of the intake air, air-fuel ratio and EGR rate; and
control means for controlling at least either the NOx purifying catalyst or the combustion state in the internal combustion engine in order to lower the amount of NOx emission.
In the device for controlling an internal combustion engine according to the present invention, the theoretical formula and the empirical formula contain a correction coefficient that varies depending upon at least either the model of the internal combustion engine or the combustion mode.
In the device for controlling an internal combustion engine according to the present invention, the combustion mode includes a stratified combustion mode and a homogeneous combustion mode.
In the device for controlling an internal combustion engine according to the present invention, the NOx operation means estimates the oxygen concentration, nitrogen concentration and temperature of the combustion gas in the internal combustion engine from the theoretical formula and the empirical formula, and estimates the amount of NOx emission in the exhaust gas based upon the oxygen concentration, nitrogen concentration and temperature of the combustion gas.
In the device for controlling an internal combustion engine according to the present invention, the control means controls the air-fuel ratio to control the NOx purifying catalyst.
In the device for controlling an internal combustion engine according to the present invention, the control means controls at least one of the fuel injection amount, fuel injection timing, ignition timing and EGR rate of the internal combustion engine as the combustion state of the internal combustion engine.
In the device for controlling an internal combustion engine according to the present invention, the air-fuel ratio detector means includes:
an air-fuel ratio sensor provided in the exhaust pipe upstream of the NOx purifying catalyst and for producing an oxygen concentration detection signal depending upon the oxygen concentration in the exhaust gas; and
air-fuel ratio operation means for estimating the air-fuel ratio based upon the oxygen concentration detection signal.
In the device for controlling an internal combustion engine according to the present invention, the air-fuel ratio detector means includes air-fuel ratio operation means for estimating the air-fuel ratio from the fuel injection amount and from the intake air amount of the internal combustion engine.